Three lens apochromatic objective of two components



btAKUH our H. KOHLER March 19, 1957 mass was APOCHROIMTIC OBJECTIVE OF'rwo COMPONENTS 7 2 3 5 3 Filed April 27, 1955 United States PatentTHREE LENS APOCHROMATIC OBJECTIVE OF TWO COMPONENTS Horst Kiihler,Heidenheim (Brenz), Germany, assignor to Carl Zeiss, Heidenheim (Brenz),Germany Application April 27, 1955, Serial No. 504,279

Claims priority, application Germany May 3, 1954 2 Claims. (Cl. 88-57)The present invention is relative to an improvement of three-lensedapochromatic objectives, comprising two converging lenses, which enclosea diverging lens. Apochromatic objectives containing three lenses havebeen known for a long time. Objectives of this type are corrected forthe image defects of spherical longitudinal deviation, sine condition,chromatic longitudinal deviation for two colours of the spectral rangeand also for a third colour, the so-called secondary spectrum.Neglecting the thicknesses, the following equation as a condition forobtaining the chromatic correction results from the theory for theelimination of the chromatic longitudinal aberration for two colours:

the resulting power of refraction of same, the following equation may beset:

For the correction of the secondary spectrum, i. e. the elimination ofthe colour longitudinal aberration for a third colour, beside Equations1 and 4 the following equation must govern:

# 1 1 wi h 1'1. +2 1 V1 11 In the above, 5 6 and 8 are the relativepartial dispersions of lenses 1, II and III. If these relative partialdispersions are related to colour G as third colour, the followingresults:

11 12 11 being the refractive figures for the Fraunhofer lenses C, F, G.0 in (5) designates the resulting relative partial dispersion of the twoconvergent lenses I and III.

, A precondition for satisfying Equation 5 is the requirement' that therelative partial dispersions of all three lenses are not allowed toanswer one and the same linear relation of 6=A +Bv (7) i. e. for one ofthe three lenses it must be possible to show relative partial dispersionwith another constant than applicable to the other two lenses independence from v.

In order to satisfy these requirements the so-called short-flint glassesin combination with crown glasses I =total power of refraction requiredof the objective) also must be satisfied, the use of glass types forapochromats hitherto employed means that the absolute amounts of thepower of refraction of (p (p (p would have to be very large incomparison to 1 This resulted in the fact that these apochromaticobjectives had a strong Gauss error (chromatic difference of sphericalaberration) and a strong zonal aberration of the spherical longitudinalaberration. Serviceable apochromats of this kind have therefore beenknown only for apertures smaller than 1:10. The invention is based onthe discovery made upon systematically examining optical glasses moltenin the past years that especially the more recent heavy flint glasses"with a refractive index of greater than 1.61 and a v-value smaller than35 also show d-values whose slope deviates from the normal linearfunction A-i-Bv of the other optical glasses.

The invention consists in that in a three-lens apochromatic objectiveconsisting of two convergent lenses which enclose a divergent lens, theconvergent lens placed as rear lens is cemented to said divergent lensand said cemented component is separated from the other said convergentlens by an air space, that furthermore the one said convergent lens aswell as the said divergent lens are made of glasses whose Abb number 11are smaller than 35 and whose relative partial dispersion are greaterthan 1.61 and that finally the divergent lens with respect to the one ofboth said convergent lenses has a difference in the refractive index ofless than 0.05 and with respect to the other said convergent lens ofmore than 0.1 for the yellow helium line d.

For reasons of correcting the image aberrations-viz. of the longitudinalspherical aberration, of the ofience against the sine condition, and ofthe chromatic aberrations named above, as well as for reasons ofreducing some manufacturing difliculties inherent in cemented triplets,it has prooved especially suitable, according to the invention, tocement the divergent lens merely with one of said convergent lenses andto leave an air space between said cemented doublet and the other saidconvergent lens such that the single convergent lens is placed in frontposition of the system regarding the incidence of light. An especiallygood correction of the chromatic aberrations results in that, accordingto the further development of the principle of the invention, the singleconvergent lens is a lens with the lower refractive index, and that thisindex is greater than 1.6 and the Abb number is greater than 55. 7

According to the invention it suflices if the air space is greater than2% of the focal length of the objective. Preferably it may amount toapproximately 6% of this focal length. Likewise these modifications,according to the invention, result in a shortening of the total lengthof the telescope, in which the objective is used.

The numerical values given in the table below are relative to an exampleof an objective according to the invention, which is also illustrated inthe figure of the drawing herewith attached. In other modifications ofthe invention, beside this numerical example, modifications may have e.g. the single convergent element made of glass of a higher refractiveindex than the convergent element cemented to the divergent element.

In the following numerical example there are designated By r the radiiof the refracting surfaces,

By d the thicknesses of the individual lens elements L1,

L2 and L3,

By I the airspace between lens elements L1 and L2,

By s the intercept from the lens apex nearest the image to the focalplane 1. Three-lens apochromatic objective consisting of two convergentlenses which enclose a divergent lens of biconcave shape, the one saidconvergent lens located at the rear position of the objective beingcemented with said divergent lens and said front cemented doublet beingseparated from the said front convergent lens of biconvex shape by anair space, the said rear convergent lens as well as the said divergentlens being made of glasses whose Abb number u is smaller than 35 andwhose relative partial dispersion is greater than 1.61, the saiddivergent lens with respect to the said rear convergent lens having adifierence in the refractive index of less than 0.05 and with respect tothe said front convergent lens of more than 0.1 for the yellow heliumline :1, the said single front convergent lens being made of a glasshaving lower refractive index than either of said lens elements of saidcemented doublet and amounting to at most 1.6 and the Abb number of theglass amounting to at least 55, and the radii (r1 as) of the refractivesurfaces of the individual lens elements beginning with the frontsurface of the said single convergent lens having values lying betweenand the said air space (I) amounting to between 0.02-,f l 0.06-f

1 being the focal length of the objective.

2. An objective according to claim 1, the surface refractive powers(An/r) deviating each by at most 102/ f from the values to be taken fromthe following numerical example:

Radii Thielcuesses nu v4 An/r d1=0. 057- f l. 6204 60. 3

ra= O. 55431- f 1. 3624/! ds=0. 047- f 1. 7847 25. 7

wherein are designated With r the radii of the refractive surfaces,

With d the thicknesses of the individual lens elements L1, L2 and La,

With 1 the airspace between lens elements L1 and L2,

With s the intercept from the lens apex nearest the image to the focalplane,

With ml the refractive indices,

With An/r the mean refractive powers of the individual lens surfaces.

References Cited in the file of this patent UNITED STATES PATENTS415,040 Hastings Nov. 12, 1889 554,737 Schroeder Feb. 18, 1896 576,896Rudolph Feb. 9, 1897 635,473 Goerz et al. Oct. 24, 1899 682,017 AldisSept. 3, 1901 775,353 Von Rohr Nov. 22, 1904 2,417,942 Miles Mar. 25,1947

